The various features and multiple structural levels of semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition and other parameters of nanoscale structures.
As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult. Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty. For example, modern memory structures are often high-aspect ratio, three-dimensional structures that make it difficult for optical radiation to penetrate to the bottom layers. In addition, the increasing number of parameters required to characterize complex structures (e.g., FinFETs), leads to increasing parameter correlation. As a result, the parameters characterizing the target often cannot be reliably decoupled with available measurements. In another example, opaque, high-k materials are increasingly employed in modern semiconductor structures. Optical radiation is often unable to penetrate layers constructed of these materials. As a result, measurements with thin-film scatterometry tools such as ellipsometers or reflectometers are becoming increasingly challenging.
In response, more complex optical tools have been developed. For example, tools with multiple angles of illumination, shorter and broader ranges of illumination wavelengths, and more complete information acquisition from reflected signals (e.g., measuring multiple Mueller matrix elements in addition to the more conventional reflectivity or ellipsometric signals) have been developed. However, these approaches have not reliably overcome fundamental challenges associated with measurement of many advanced targets (e.g., complex 3D structures, structures smaller than 10 nm, structures employing opaque materials) and measurement applications (e.g., line edge roughness and line width roughness measurements).
Atomic force microscopes (AFM) and scanning-tunneling microscopes (STM) are able to achieve atomic resolution, but they can only probe the surface of the specimen. In addition, AFM and STM microscopes require long scanning times. Scanning electron microscopes (SEM) achieve intermediate resolution levels, but are unable to penetrate structures to sufficient depth. Thus, high-aspect ratio holes are not characterized well. In addition, the required charging of the specimen has an adverse effect on imaging performance.
To overcome penetration depth issues, traditional imaging techniques such as TEM, SEM etc., are employed with destructive sample preparation techniques such as focused ion beam (FIB) machining, ion milling, blanket or selective etching, etc. For example, transmission electron microscopes (TEM) achieve high resolution levels and are able to probe arbitrary depths, but TEM requires destructive sectioning of the specimen. Several iterations of material removal and measurement generally provide the information required to measure the critical metrology parameters throughout a three dimensional structure. But, these techniques require sample destruction and lengthy process times. The complexity and time to complete these types of measurements introduces large inaccuracies due to drift of etching and metrology steps. In addition, these techniques require numerous iterations which introduce registration errors.
Another response to recent metrology challenges has been the adoption of x-ray metrology for measurements including film thickness, composition, strain, surface roughness, line edge roughness, and porosity.
Small-Angle X-Ray Scatterometry (SAXS) systems have shown promise to address challenging measurement applications. Various aspects of the application of SAXS technology to the measurement of critical dimensions (CD-SAXS) and overlay (OVL-SAXS) are described in 1) U.S. Pat. No. 7,929,667 to Zhuang and Fielden, entitled “High-brightness X-ray metrology,” 2) U.S. Patent Publication No. 2014/0019097 by Bakeman, Shchegrov, Zhao, and Tan, entitled “Model Building And Analysis Engine For Combined X-Ray And Optical Metrology,” 3) U.S. Patent Publication No. 2015/0117610 by Veldman, Bakeman, Shchegrov, and Mieher, entitled “Methods and Apparatus For Measuring Semiconductor Device Overlay Using X-Ray Metrology,” 4) U.S. Patent Publication No. 2016/0202193 by Hench, Shchegrov, and Bakeman, entitled “Measurement System Optimization For X-Ray Based Metrology,” 5) U.S. Patent Publication No. 2017/0167862 by Dziura, Gellineau, and Shchegrov, entitled “X-ray Metrology For High Aspect Ratio Structures,” and 6) U.S. Patent Publication No. 2018/0106735 by Gellineau, Dziura, Hench, Veldman, and Zalubovsky, entitled “Full Beam Metrology for X-Ray Scatterometry Systems.” The aforementioned patent documents are assigned to KLA-Tencor Corporation, Milpitas, Calif. (USA).
Research on CD-SAXS metrology of semiconductor structures is also described in scientific literature. Most research groups have employed high-brightness X-ray synchrotron sources which are not suitable for use in a semiconductor fabrication facility due to their immense size, cost, etc. One example of such a system is described in the article entitled “Intercomparison between optical and x-ray scatterometry measurements of FinFET structures” by Lemaillet, Germer, Kline et al., Proc. SPIE, v.8681, p. 86810Q (2013). More recently, a group at the National Institute of Standards and Technology (NIST) has initiated research employing compact and bright X-ray sources similar those described in U.S. Pat. No. 7,929,667. This research is described in an article entitled “X-ray scattering critical dimensional metrology using a compact x-ray source for next generation semiconductor devices,” J. Micro/Nanolith. MEMS MOEMS 16(1), 014001 (January-March 2017).
SAXS has also been applied to the characterization of materials and other non-semiconductor related applications. Exemplary systems have been commercialized by several companies, including Xenocs SAS (www.xenocs.com), Bruker Corporation (www.bruker.com), and Rigaku Corporation (www.rigaku.com/en).
Many x-ray metrology techniques used in semiconductor manufacturing can benefit from high brightness x-ray sources. For example, critical dimension small angle x-ray scattering (CD-SAXS) measurements often require long integration times due to the low scattering of certain materials. A high brightness source can improve the throughput of CD-SAXS measurements.
Development efforts in the area of extreme ultraviolet (EUV) lithography are focused on light sources that emit narrowband radiation (e.g., +/−0.1 nm) centered at 13 nanometers (i.e., 92.6 electron volts) at high power levels (e.g., 210 watts of average power at the intermediate focus of the illuminator). Light sources for EUV lithography have been developed using a laser droplet plasma architecture. For example, xenon, tin, and lithium droplet targets operating at pulse repetition frequencies of approximately 100 kHz are pumped by CO2 coherent sources. The realized light is high power (e.g., 210 watts of average power at the intermediate focus of the illuminator is the goal for lithography tools at 13 nanometers). However, the resulting radiation is relatively low energy (92.6 electron volts), which severely limits the utility of these illumination sources in metrology applications. An exemplary system is described in U.S. Pat. No. 7,518,134 to ASML Netherlands B.V., the content of which is incorporated herein by reference in its entirety.
In some examples, x-ray illumination light is generated by high energy electron beam bombardment of a solid target material, such as rotating anode target material. Rotating anode X-ray sources are commonly employed for medical imaging and analytical chemistry applications. Numerous versions of rotating anode X-ray sources are manufactured by companies such as Philips, General Electric, Siemens, and others, for medical imaging applications such as tomography, mammography, angiography, etc. Rigaku Corporation and Bruker Corporation manufacture continuously operated rotating anode sources for analytical chemistry applications such as X-Ray diffraction (XRD), X-Ray Reflectometry (XRR), small angle X-Ray scatterometry (SAXS), wide angle X-Ray scatterometry (WARS), etc.
Rotating anode targets enable more effective heat removal from the anode material compared to stationary anode targets. Continuously moving the location of electron beam impingement on the anode surface results in convective heat dissipation that decreases focal spot impact temperature and improves X-ray tube power loading capability. A typical rotating anode source rotates anode material at 5,000-10,000 revolutions per minute, or higher. The linear speed of the anode material at the focal spot location may be 100 meters/second, or higher.
Improvements directed toward increased anode heat dissipation and thermal conductivity have been proposed. For example, the FR-X model X-ray sources manufactured by Rigaku Corporation (Japan) and the MicroMax model X-ray sources manufactured by Bruker AXS GmbH (Germany) employ water cooling to dissipate heat generated at the anode.
U.S. Pat. No. 9,715,989 describes a rotating anode structure with high thermal conductivity diamond layers. U.S. Pat. No. 8,243,884 describes the use of diamond-metal composite materials to improve heat dissipation. U.S. Pat. No. 7,440,549 describes a rotating anode device that dissipates heat by a heat pipe effect. U.S. Patent Publication No. 2015/0092924 describes a microstructural anode including a high atomic number material embedded in a high thermal conductivity matrix. U.S. Pat. No. 9,159,524 and U.S. Pat. No. 9,715,989 describe similar diamond-based heat management solutions in the context of stationary anode sources. The contents of the aforementioned U.S. Patents and U.S. Patent Publications are incorporated herein by reference in their entirety.
Despite improved power loading capabilities, rotating anode sources suffer from significant limitations. In practice, microcracks form at the surface of the solid anode material located on the focal track (i.e., the locus of points repeatedly subjected to e-beam impingement) due to repeated thermal cycling. These microcracks introduce losses due to increased absorption. In some examples, a 20-30% drop in X-ray flux occurs within the first 1,000 hours of source operation. In addition, a typical rotating anode requires re-polishing (i.e., restoration of the surface of the anode material) approximately every 3,000 hours. In addition, in some examples, high rotation speeds limit X-ray spot size and spatial stability of the X-ray spot.
In some other examples, x-ray illumination light is generated by high energy electron beam bombardment of a liquid target material to mitigate the formation of surface microcracks associated with solid anode targets.
In some of these examples, a liquid metal jet anode is employed. An exemplary liquid metal jet x-ray illumination system is described in U.S. Pat. No. 7,929,667 to Zhuang and Fielden, the content of which is incorporated herein by reference in its entirety. Another exemplary liquid metal jet x-ray illumination source is described in U.S. Pat. No. 6,711,233, the content of which is incorporated herein by reference in its entirety. The liquid metal jet effectively refreshes the anode surface continuously to eliminate the formation of surface microcracks. However, the liquid metal anode material does evaporate and form a metal vapor that may limit x-ray source lifetime. In some examples, the metal vapor condenses on the vacuum x-ray window causing additional x-ray absorption. In some examples, the metal vapor diffuses into the cathode region and contaminates the cathode, reducing cathode lifetime and system output. In some examples, the metal vapor diffuses into the electron beam acceleration region causing high-voltage breakdowns.
In some other examples, a liquid metal anode is flowed over a stationary structure. U.S. Pat. No. 4,953,191 describes a liquid metal anode material flowing over a stationary metal surface, the content of which is incorporated herein by reference in its entirety. U.S. Pat. No. 8,629,606 describes a liquid metal anode material flowing on internal surfaces of an X-ray source vacuum enclosure, the content of which is incorporated herein by reference in its entirety. U.S. Patent Publication No. 2014/0369476 and U.S. Pat. No. 8,565,381 describe a liquid metal anode material flowing through a channel or tube, the content of each is incorporated herein by reference in its entirety. The fast moving liquid metal is enclosed in part by suitable windows to allow electron beam penetration and X-ray extraction.
Despite improved power loading capabilities, liquid anode sources suffer from significant limitations. In practice, flowing thin liquid metal layers over other surfaces is limited to relatively low velocity flow. As flow velocity increases, turbulence arises, which destabilizes the X-ray illumination source. As a result, anode power loading of an X-ray source employing liquid anode material flowing over another surface is significantly limited. In addition, anode power loading for X-ray illumination sources based on flowing liquid metal inside channels and tubes is limited by the structural integrity of any windows employed to contain the flow and allow electron beam penetration and X-ray extraction.
Similarly, stable operation of a liquid metal jet X-ray illumination source requires a laminar liquid metal jet flow. Therefore, any increase in jet speed to accommodate increased anode power loading is limited by the laminar-turbulent transition of the jet itself and the feasibility of an ultra-high-pressure jet return loop required to achieve any increased jet velocity.
Unfortunately, X-ray based metrology throughput is impaired by limited power loading on the anode. An increase in power loading of a conventional solid metal anode source causes ablation and destruction of the anode. For typical liquid metal sources, an increase in power loading tends to destabilize the X-ray illumination source.
Future metrology applications present challenges for metrology due to increasingly high resolution requirements, multi-parameter correlation, increasingly complex geometric structures, and increasing use of opaque materials. The adoption of x-ray metrology for semiconductor applications requires improved x-ray sources with the highest possible brightness.